Catalysts play many important roles in industry. One such role is fluid conditioning, including decontamination of flowing fluid. For example, a catalytic system might be employed to remove oxygen content from an inert gas flow. The “catalytic converter” employed in automobiles (circa. 2007 and earlier) removes certain pollutants from an exhaust flow produced by the engine. “Solid” catalysts—so-called because the catalytic compound exists in solid phase during use—are often employed in this role, as fluid decontamination typically involves removal of the contaminant from fluid phase, aerosol state, solution, or entrainment.
“Raney nickel”, a solid phase catalyst formed of nickel grains bonded in a skeletal structure along with aluminum grains, performs many industrial roles, including fluid conditioning. A variety of similar catalysts employ other active materials, including iron or copper, instead of nickel, and other alloying components, such as zinc or silicon, instead of aluminum. Currently, all of these “Raney-style” catalysts are formed via processes essentially similar to the original recipe for Raney nickel.
A Raney-style process describes a multi-step method of forming a porous, active metal catalyst. First, a precursor is formed of at least a binary alloy of metals where one of the metals can be extracted. Second, the precursor is activated by extracting an alloy constituent leaving a porous residue comprising a metal that has catalytic activity. Such processes are described in, e.g. Raney, M. Catalysts from Alloys, Ind. Eng. Chem., 1940, 32, 1199; as well as U.S. Pats. Nos. 1,628,190; 1,915,473; 2,139,602; 2,461,396 and 2,977,327 to M. Raney. Commercial catalysts made by these type of processes are sold by W. R. Grace & Co. under the trademark RANEY® catalyst.
Often, additional materials are added and process parameters are varied to achieve a desired catalytic activity or function. Typically, the process parameters and additional materials included depend both on the active material employed and the catalytic function desired. Some added materials called “promoters” serve to enhance catalytic activity. A typical process parameter that is varied according to specific needs is the precursor alloy composition. For example, the precursor used for Raney nickel typically consists of equal amounts of nickel and aluminum by volume.
The traditional Raney-style process results in a collection of granular pieces, each with an internal porosity. Depending on their grain size, these particles are used in slurry or in packed-column systems as heterogeneous catalysts. Generally, larger particle sizes are required for use in packed-column systems. Traditionally, there is a tradeoff between surface area and particle size, with larger-sized particles having less surface area per unit volume. See, e.g. the background section of U.S. Pat. No. 4,895,994.
Although small powder catalysts have desirable surface area to volume characteristics, they are only suitable for batch processing and must be isolated after use. In order to avoid these disadvantages, a variety of processing regimes have been proposed to permit use of Raney particles in fixed-bed catalysis. For example, U.S. Pat. No. 4,895,994 describes a fixed bed catalyst shaped from Raney precursor mixed with a polymer, cured, and then activated via a leaching process. U.S. Pat. No. 5,536,694 describes a fixed-bed catalyst prepared from powders of Raney precursor mixed with a powder of its catalytically active component as a binder. However, these processes involve high sintering temperatures and thus cannot accommodate small, high surface-to-volume-ratio precursor particles (the sintering temperatures are sufficient to destroy the grain structure of the precursor alloy in small particles). Thus, lacking the high surface to volume ratio provided by the smallest precursor sizes, these approaches instead rely on macroporosity of the fixed bed structure to achieve high internal diffusion, making the most of their surface area.
Therefore, the smallest precursor particles suitable for fixed-bed catalyst production via traditional means are micron scale particles. FIG. 1 illustrates such a system process. Micron scale aluminum powder 112 and micron scale nickel powder 114 are combined in a melt-based alloying step 116, thereby producing nickel-aluminum alloy 118 in a variety of alloy phases. The nickel-aluminum alloy 118 is then processed and activated, such as by a leaching apparatus 120, resulting in a bulk porous structure 122 that is mostly nickel (though some aluminum may remain). In some prior art systems, the activation precedes a processing step, whereas in other systems, the activation follows processing to produce a bulk structure. In either case, an aluminum leach step 120 is involved in producing a bulk porous structure 122 composed substantially of nickel. Unfortunately, the smallest pores within the structure produced are micron scale.
What is needed in the art is a system and method for producing a catalyst precursor material and a skeletal catalyst having smaller particle size, and therefore larger surface area available for catalysis.